1. Field of the Invention
The present invention relates to a reference fixture for a roundness measuring instrument, particularly, the one that can acquire origin information of the roundness measuring instrument and calibrate a detector thereof.
2. Description of Related Art
Conventionally, a roundness measuring instrument, which is one type of form measuring instrument, have been used as a measuring instrument that measure geometrical deviation such as roundness, flatness and parallelism of an object to be measured.
Such roundness measuring instrument has a detector provided with a sensor at a distal end of a lever capable of swinging in a direction orthogonal to the surface of a workpiece, or the object to be measured. A measurement data is acquired by the detector scanning along the surface of the workpiece with the sensor contacting the surface of the workpiece and collecting the data so as to measure roundness, cylindricity, straightness, coordinate or dimension of the workpiece.
In the above roundness measuring instrument, in order to measure the workpiece accurately and smoothly, setting for an origin of the roundness measuring instrument and sensitivity calibration of the detector are preferably completed before starting the measurement.
A first method known for setting the origin to acquire a coordinate value of a workpiece is that which performs form analysis of a scanning locus and sets an origin based on a cross point of a form element (e.g., JP2002-270307 (JP2004-10883 A)).
There has been a second method that arranges a reference ball (origin ball) on a workpiece rotary mechanism provided with a turntable, which is rotatable with a workpiece being set thereon, so as to acquire origin information by measuring the surface of the origin ball. In such case, the origin ball is fixed on a shank perpendicularly disposed on the workpiece rotary mechanism. In the second method, for calibrating the detector, a calibration master is provided on the workpiece rotary mechanism, so that sensitivity error of the detector is detected for compensating the error by measuring the calibration master.
At the origin setting according to the above publication, since a calculation process on the basis of the form analysis is complicated, the calculation process takes a long time whereas a large calculation program needs to be built.
In the second method, since the shank holding the origin ball is perpendicularly provided on the workpiece rotary mechanism, the orientation of the detector becomes upward, thus being impossible to measure a bottom face of the origin ball. Therefore, data cannot be acquired from the face unavailable for the measurement, thereby impossible to acquire highly accurate origin information.
In the second method, the calibration master and the origin ball each are provided as separate components. Prior to CNC (Computer Numerical Control) measurement, machine origin information is acquired with use of the origin ball, and then, the sensitivity of the detector is adjusted with use of the calibration master in addition to the acquisition for the origin information. At this time, an operator holds the origin ball by hand to set on the workpiece rotary mechanism, removes the origin ball from the workpiece rotary mechanism after acquiring the origin information, sets the calibration master on the workpiece rotary mechanism by hand and removes the calibration master after the sensitivity adjustment of the detector is completed. Thus, since the origin ball and the calibration master need to be replaced, the preparation takes a long time and is inconvenient in use.
When the origin ball and the calibration master are replaced, since the operator touches the calibration master by hand, an affection on the calibration master due to body temperature of the operator may be the bottleneck in improvement of measuring accuracy.